Any radar equipment used by civil or military authorities for locating aircraft, ships, or ground vehicles; most commonly, in civil air-traffic management, a radar used by controllers to indicate the position of aircraft aloft or on the airport surface. Two types of air-traffic control surveillance radar are employed internationally. Ground-based conventional, or primary, radar operates by transmitting microwave pulses and detecting the resulting energy reflected from the aircraft body (airframe). Secondary surveillance radar employs cooperative radio receiver-transmitter units (transponders) on the aircraft.